This invention relates generally to the rotor windings of a dynamo-electric machine, and more particularly, to rotor and rotor end winding cooling in machines with concentric rotor windings.
The rotors in large gas cooled dynamo-electric machines have a rotor body which is typically made from a machined high strength solid iron forging. Axially extending radial slots are machined into the outer periphery of the rotor body at specific circumferential locations to accommodate the rotor winding. The rotor winding in this type of machine typically consists of a number of complete coils, each having many field turns of copper conductors. The coils are seated in the radial slots in a concentric pattern with, for example, two such concentric patterns in a two-pole rotor. The coils are supported in the rotor body slots against centrifugal forces by wedges which bear against machined dovetail surfaces in each slot. The regions of the rotor winding coils which extend beyond the ends of the main rotor body are called xe2x80x9cend windingsxe2x80x9d and are supported against centrifugal forces by high strength steel retaining rings. The inboard end of each retaining ring is typically shrunk onto a machined surface at the end of the rotor body and the outboard end of each retaining ring is typically shrunk onto a circular steel centering ring. The section of the rotor shaft forging which is disposed underneath the rotor end windings is referred to as the spindle. For ease of reference and explanation hereinbelow, the rotor winding can be characterized has having a slot end region within the radial slots at the end of the rotor body, and a rotor end winding region that extends beyond the pole face, radially spaced from the rotor spindle. This invention relates primarily to the cooling of the slot end region and of the rotor end winding region.
In generator rotor designs with Diagonal Flow and Radial Flow cooling, the rotor end windings are generally the hottest part of the field winding because these regions are difficult to cool effectively. Heat is difficult to remove because the mechanical structure that supports the end winding under high centrifugal loads inhibits the placement of cooling passages, lest the mechanical integrity of the support system be compromised. Consequently, rotor end windings are cooled either passively (sometimes referred to as xe2x80x9cfree convectionxe2x80x9d) or by forced convection, which introduces cooling gas directly into long passages in the copper field turns. Passive cooling systems, while simple in construction, have inherently low heat transfer effectiveness. The forced convection passages carry cooling gas longitudinally along the copper field turn for significantly long distances until the hot gas can be discharged into the air gap (i.e., the gap between the rotor and stator) through holes in the wedges in the main body section of the rotor, inboard of the retaining ring support structure. The transport temperature rise of the cooling gas causes the gas to be less effective in removing heat as it progresses along these long passages. In fact, the cooling gas entrapped in long passages can reach a temperature where it can no longer maintain the copper conductors within required thermal limits. In such cases, a second set of cooling grooves may be introduced at an intermediate point along the length of the copper field turn; either in the same turn if space and current carrying capacity allows, or in alternative layers of the copper turns. An example of such a cooling scheme is disclosed in U.S. Pat. No. 4,709,177, the entire disclosure of which is incorporated herein by this reference. In this configuration, some turns have long full-length passages while others have the shorter passages whose entrance ports are closer to the discharge. In either case, the hot gas is transported through the entire length of the long passage, resulting in locally elevated operating temperatures in the region approaching the discharge holes. Therefore, there is a need for a more effective cooling scheme in the end regions of electrical generator rotors.
Thus, several previous attempts at reducing hot spot temperatures in field windings have been made. Early attempts included drawing in gas from the ends of the rotor between the coils and discharging it through vent slots in the teeth. The large spacing between coils made this form of cooling relatively ineffective. Later schemes involved directing gas through long channels in the copper and discharging them through a chimney in the body section of the rotor. However, these suffer from the need for long ducts, as described above. As the gas flows along the channel, it picks up heat and becomes less effective at reducing local copper temperature. This introduces a practical limit to how long the grooves can be.
To address the deficiencies of the prior art cooling/ventilating schemes, shorter, independent cooling passages are provided in the slot end region. This lowers the rotor field winding operating temperature and makes it more uniform. To provide for these shorter independent cooling passages, the hot end winding gas is discharged into passages or channels between the windings for discharge. Most preferably, the discharged hot end winding gas is directed via cavities in body support blocks to vent slots or holes in the rotor teeth, thereby to define a substantially uninterrupted flow path from the end winding grooves to the air gap between the stator and rotor. Routing the cooling gas through such support blocks provide an additional, parallel flow path from the end grooves to the air gap which increases the cooling capacity of the end winding. Thus, the slot end region of the copper turns can be ventilated independently from the end winding by providing a second groove which introduces the cold gas just inboard of the end turn discharge ports.
Thus, the rotor end winding cooling scheme of the invention ventilates the slot end region of the turns separately from the coil end region. This reduces the length of the individual ventilation ducts in the conductors which will in turn reduce hot spots in the rotor winding. Vent slots in the rotor teeth may be used to discharge the hot coil side gas.
In its broadest respects, therefore, the present invention relates to a cooling gas ventilation circuit for an end winding of a rotary machine having a rotor, and a plurality of coils seated in radial slots provided in the rotor, the coils each comprising a plurality of radially stacked turns, the coils extending beyond a pole face of the rotor to form an end winding. The ventilation circuit is composed of first and second cooling gas passages, each respectively defined in at least one turn of each coil of the end winding. Each first cooling gas passage extends from an inlet port in communication with a cavity on one longitudinal side of the turn to an exit port defined on the other longitudinal side of the turn. Each second cooling gas passage extends from an inlet port in communication with the cavity on the one longitudinal side of the turn to an outlet in the form of a radial chimney defined through a plurality of the turns of the coil within the respective radial slot. In the presently preferred embodiment, the second passage is longitudinally offset with respect to the first cooling passage, so they serve to cool respective portions of the coil. The first and second passages may be but are not necessarily defined in the same turn of their respective coil.